Microsoft SQL Server/Using Instances

On one machine, there can be more editions, as defined in the SQL Server Installation Center.

Default Instance: Only one. Please note some of the MS products using the SQL Server request the default instance (For instance the Team Foundation Server).

Determining Multiple or Single Instances
Multiple Instances mean more administration overhead. Additional Instances require additional resources: Memory and processor capacity-

Advantages of multiple instances
 * testing multiple versions
 * testing service packs, dev databases and applications
 * different customers require their own system and user databases with full administrative control or their instance
 * desktop engine is embedded in the applications, because every application can install each own instance